1. Field of the Invention
The present invention relates to a process for carbonitriding a steel comprising:
0.75-1.1% by weight of C; PA1 up to 1.0% by weight of Si; PA1 less than 0.015% by weight of P; PA1 up to 0.5% by weight of Mo; PA1 up to 1.2% by weight of Mn; PA1 0.5-2% by weight of Cr, PA1 the remainder being Fe, PA1 0.75-1.1% by weight of C; PA1 up to 1.0% by weight of Si; PA1 less than 0.015% by weight of P; PA1 up to 0.5% by weight of Mo; PA1 up to 1.2% by weight of Mn; PA1 0.5-2% by weight of Cr; PA1 the remainder being Fe,
by exposing it at elevated temperatures to an atmosphere comprising at least carbon monoxide, hydrogen and ammonia. Such a steel grade is disclosed by U.S. Pat. No. 4961904. Such steel grades are used, for example, for rings and rolling elements of rolling bearings. For general applications, this steel is found to be particularly suitable for roller bearings. It has been found, however, that for certain applications in which a high static load arises, the steel has to be subject to more stringent requirements. Relevant examples include bearings used in gearboxes of vehicles and inalternators.
2. Description of the Prior Art
It has been proposed in the prior art to employ, in order to obtain highly alloyed martensite grades, either a steel grade having a high carbon content or to employ higher hardening temperatures or to apply a combination of these two. This does, however, have the drawback that the structure does not remain fine-grained, the toughness decreases and the amount of retained austenite increases, as a result of which dimensional stability decreases.